Skip to content

Page203

SDLC and HDLC

Synchronous Data Link Control (SDLC) is a synchronous Layer 2 WAN protocol that uses polling to transmit data. Combined nodes can act as primary or secondary. SDLC supports NRM transmission only (see below).

High-Level Data Link Control (HDLC) is the successor to SDLC. HDLC adds error correction and flow control, as well as two additional modes (ARM and ABM). The three modes of HDLC are:

  • Normal Response Mode (NRM)—Secondary nodes can transmit when given permission by the primary
  • Asynchronous Response Mode (ARM)—Secondary nodes may initiate communication with the primary
  • Asynchronous Balanced Mode (ABM)—Combined mode where nodes may act as primary or secondary, initiating transmissions without receiving permission

Converged Protocols

“Convergence” is a recent network buzzword. It means providing services such as industrial controls, storage, and voice (that were typically delivered via non-IP devices and networks) via Ethernet and TCP/IP.

DNP3

The Distributed Network Protocol (DNP3) provides an open standard used primarily within the energy sector for interoperability between various vendors’ SCADA and smart grid applications. According to the US Department of Energy, “Smart grid” generally refers to a class of technology people are using to bring utility electricity delivery systems into the 21st century, using computer-based remote control and automation. These systems are made possible by two-way communication technology and computer processing that has been used for decades in other industries. They are beginning to be used on electricity networks, from the power plants and wind farms all the way to the consumers of electricity in homes and businesses. They offer many benefits to utilities and consumers—mostly seen in big improvements in energy efficiency on the electricity grid and in the energy users’ homes and offices [8].

Some protocols, such as SMTP, fit into one layer. DNP3 is a multilayer protocol and may be carried via TCP/IP (another multilayer protocol): “Many vendors offer products that operate using TCP/IP to transport DNP3 messages in lieu of the media discussed above. Link layer frames, which we have not talked about yet, are embedded into TCP/IP packets. This approach has enabled DNP3 to take advantage of Internet technology and permitted economical data collection and control between widely separated devices” [9].

Recent improvements in DNP3 allow for “Secure Authentication,” which addresses challenges with the original specification that could have allowed, for example, spoofing or replay attacks. DNP3 became an IEEE standard in 2010, called IEEE 1815-2010 (now deprecated). It allowed pre-shared keys only. IEEE 1815-2012 is the current standard; it supports Public Key Infrastructure (PKI).

Storage Protocols

Fibre Channel over Ethernet (FCoE) and Internet Small Computer System Interface (iSCSI) are both Storage Area Network (SAN) protocols that provide cost-effective ways to leverage existing network infrastructure technologies and protocols to interface with storage. A Storage Area Network allows block-level file access across a network, just like a directly attached hard drive. Note that fibre channel uses the Canadian/UK spelling of “fibre,” while fiber optic cable typically uses the American spelling of “fiber.”

FCoE leverages Fibre Channel, which has long been used for storage networking, but dispenses with the requirement for completely different cabling and hardware. Instead, FCoE can be transmitted across standard Ethernet networks. In FCoE, Fibre Channel’s HBA (Host Bus Adapters), which historically were unique cards to interface with storage, can be combined with the network interface (NIC) for economies of scale. FCoE uses Ethernet, but not TCP/IP. Fibre Channel over IP (FCIP) encapsulates Fibre Channel frames via TCP/IP.

Like FCoE, iSCSI is a SAN protocol that allows for leveraging existing networking infrastructure and protocols to interface with storage. While FCoE simply uses Ethernet, iSCSI makes use of higher layers of the TCP/IP suite for communication, and can be routed like any IP protocol (the same is true for FCIP). By employing protocols beyond Layer 2 (Ethernet), iSCSI can be transmitted beyond just the local network. Thus, iSCSI could even allow for accessing storage that resides across a WAN. iSCSI uses Logical Unit Numbers (LUNs) to provide a way of addressing storage across the network. LUNs can also be used for basic access control for network accessible storage.